The present invention relates to a method of diagnosing the efficiency of an exhaust gas stoichiometric composition sensor placed downstream of a catalytic converter.
As is known, two exhaust gas stoichiometric composition sensors (lambda probes), arranged upstream and downstream of the catalytic converter, respectively, are present on catalyzed vehicles equipped for conducting on-board diagnosis operations.
Each of the sensors is able to generate an output signal which, after suitable processing, exhibits two levels dependent on the stoichiometric composition of the exhaust gases and, consequently, on the stoichiometric composition of the air/fuel mixture supplied to the engine.
In particular, if the air/fuel mixture has more fuel than required by the stoichiometric ratio (rich mixture) the signal generated by the sensor assumes a high value (typically 800-900 mV), whereas if the air/fuel mixture has less fuel than required by the stoichiometric ratio (weak mixture) the signal generated by the sensor assumes a low value (typically 100-200 mV).
The regulations in force for vehicle emissions stipulate that a sensor should be declared faulty when its deterioration is such that it does not allow correct operation of the supply unit, so that the emissions exceed preset limits, or else is such that the sensor delivers unreliable values and cannot therefore be used to perform the required diagnoses on-board the vehicle.
Such a deterioration is manifested via a variation in the voltage levels of the output signal generated by the sensor and/or via an increase in the switching time of the sensor, defined as the delay between a variation in the stoichiometric ratio of the mixture and the corresponding change in the level of the output signal generated by the sensor.
Numerous methods of diagnosis have been developed in order to register such a deterioration. However, such methods are capable of diagnosing only malfunctions of the sensor placed upstream of the catalytic converter, because the gauging of malfunctions of the sensor placed downstream of the catalytic converter is strongly influenced by the state of deterioration of other components of the vehicle, in particular by the deterioration of the catalytic converter, which such methods are incapable of distinguishing.